The present invention relates generally to communications systems. More particularly, this invention relates to a system and method for controlling information transmission and communication handoff between frequency division multiplexing and time division multiplexing communication systems using any multiple access scheme.
The modern communications era has brought about a tremendous proliferation of wireline and wireless networks. Computer networks, television networks, and telephony networks in particular are experiencing an unprecedented technological expansion, fueled by consumer demand. The ever-increasing need for transportation, due in part to the expansion of the world-wide market and the popularity of suburbia, has led to an increased use of automobiles and airplanes for business and pleasure. The desire to maintain the ability to communicate, even while away from the home or office, has driven the wireless communication market to a large extent. One response to this demand was the mobile/wireless telephone network.
The demand by consumers all over the world for mobile communications is expanding at a rapid pace and will continue to do so for at least the next decade. Over 100 million people were using a mobile service by the end of 1995, and that number is expected to grow to 300 million by the year 2000. Several factors are contributing to the exciting growth in the telecommunications industry. For example, a combination of technology and competition bring more value to consumers. Phones are smaller, lighter, have a longer battery life, and are affordable now for the mass market. Operators are providing excellent voice quality, innovative services, and roaming across the country or world. Most important, mobility is becoming less expensive for people to use. Around the world, as well as in the United States, governments are licensing additional spectrum for new operators to compete with traditional cellular operators. Competition brings innovation, new services, and lower prices for consumers.
Cellular telephone communications systems allow users of cellular telephones to be connected to other cellular telephone users, as well as being connected to the conventional landline Public Switched Telephone Network (PSTN). Cellular telephones work by dividing geographical areas into xe2x80x9ccellsxe2x80x9d. Each cell includes a base station, which typically contains a transceiver, antenna, and dedicated lines to a Mobile Telephone Switching Office (MTSO). Adjacent cells may utilize different radio frequencies in order to prevent interference between the adjacent cells.
In TDMA or FDMA systems it is customary that each cell has at least one setup channel dedicated to signaling between the cell and cellular units within each cell, while the remaining channels are used for conversation. Each frequency channel may be re-used between cells, as long as the re-used channels are not in adjacent cells, and are far enough apart to avoid excessive interference. A network with a relatively small number of subscribers can therefore use large cells, and as demand grows, the cells may be divided into smaller cells.
Accordingly, in cellular networks, calls have to be passed as the vehicle or mobile unit moves from one cell to another. This is referred to as xe2x80x9chandoffxe2x80x9d or xe2x80x9chandoverxe2x80x9d. As a vehicle moves away from a base station, its signal strength decreases. While handoff is often implemented in mobile systems because of mobility of the users, there are other reasons for implementing a handoff. For example, a stationary terminal may perform a handoff to utilize a different resource where the alternative resource is preferred, such as in the case of an imbalance in the system load, better efficiency in the alternative resource, signal quality, ownership of resources, forced handoff, and the like.
The base station monitors the signal strength during the duration of the call. When signal strength falls below a predetermined threshold level, the network makes a request to all predetermined neighboring cells to report the signal strength of the mobile station in the vehicle. If the signal strength in the neighboring cell is stronger by a predetermined amount, then the network attempts to handoff the call to the neighboring cell.
The manner in which handoff occurs, and the relative quality of the handoff, depends largely on the channel access method utilized. These access methods are used to increase the traffic-carrying capacity and to provide access to that capacity. Many different access methods have been employed, including Frequency Division Multiple Access (FDMA), which divides the capacity into multiple frequency segments between end points. Time Division Multiple Access (TDMA) is another access method, which uses the concept of time sharing the total capacity. Still another access method is Code Division Multiple Access (CDMA), which may be based on the IS-95 industry specification. IS-95 CDMA combines new digital spread spectrum CDMA and advanced mobile phone service (AMPS) functionality into one dual-mode cellular telephone on the 800 MHz band, and can use a CDMA-only handset on the 1.9 GHz PCS band.
CDMA systems primarily differ from FDMA (Analog) and TDMA systems through the use of coded radio channels. In a CDMA system, users can operate on the same radio channel simultaneously by using different coded sequences. IS-95 CDMA cellular systems have several key attributes that are different from other cellular systems. The same CDMA radio carrier frequencies may be optionally used in adjacent cell sites, which eliminates the need for frequency planning.
In AMPS cellular systems, handoff occurs when the base station detects a deterioration in signal strength from the mobile station. As AMPS subscribers approach handoff, signal strength may vary abruptly and the voice is muted for at least 200 milliseconds in order to send control messages and complete the handoff. In contrast, CDMA uses a unique soft handoff, which is nearly undetectable and loses few if any information frames. As a result, CDMA""s soft handoff is much less likely to lose a call during handoff.
For providing duplex communication, transmission techniques such as Time Division Duplex (TDD) and Frequency Division Duplex (FDD) have been used. FDD provides for forward link (downlink) and reverse link (uplink) channel communications in different frequency bands. In TDD, a single channel is shared in time to carry both the transmit and receive information virtually simultaneously to achieve full duplex operation. Typically, FDD is used in outdoor systems, and TDD is used in indoor systems, or whenever local coverage is needed, due to the slow varying nature of the propagation channel and channel reciprocity.
TDD and FDD are currently not utilized in the same system. As previously described, CDMA systems, for example, use a xe2x80x9csoft handoffxe2x80x9d, which is a call state where two or more base stations support a mobile station. This, however, differs from a handoff between a TDD and an FDD system. In future systems, TDD and FDD may be optional features of one system. Therefore, there is a need to support handoffs between TDD and FDD systems in a wireless environment. The present invention provides for seamless handoff between TDD and FDD systems, and offers other advantages over the prior art.
The present invention is directed to a system and method for controlling communication handoff between frequency division multiplexing and time division multiplexing communication systems using any multiple access scheme.
In accordance with one embodiment of the invention, a method for controlling mobile unit communication handoffs between a frequency division duplex (FDD) communication system and a time division duplex (TDD) communications system is provided. The mobile unit transmits information via a current one of the FDD and TDD communication systems while it moves toward the other, or targeted one of the FDD and TDD communication systems. A pilot search signal or any other cell identification signal is generated by the targeted communication system, where the pilot search signal corresponds to a transmission range of the targeted communication system. A communication handoff is initiated from the current communication system to the targeted communication system when the mobile unit recognizes a predetermined threshold level of the pilot search signal. Concurrent communications are synchronized between the mobile unit and both the current and targeted communications systems. When the handoff is complete, transmission between the mobile unit and the current communication system is terminated, while communication between the mobile unit and the targeted communication system is maintained when the handoff is complete.
In accordance with another embodiment of the invention, the synchronization includes an initial synchronization of the handoff, and further includes temporary operation in a multirate mode. The initial synchronization includes increasing the data rate during an uplink frame with the system currently in operation, while transmitting a synchronizing preamble to the targeted communication system in the remaining uplink frame time which was made available by the increased data transfer rate with the current communication system. Communications enters the multirate mode upon acknowledgment of the synchronization by the targeted communication system. The multirate mode includes communicating the information with the current communication system at an increased data rate in the uplink frame, and concurrently communicating the information with the targeted communication system at an increased data rate in a remaining portion of each uplink frame.
In accordance with another aspect of the invention, a system for managing mobile unit communication handoffs between a frequency division duplex (FDD) communication system and a time division duplex (TDD) communication system is provided. An FDD base station is provided within the FDD communication system for communicating with the mobile unit in frequency division duplex mode, and for generating a pilot search signal corresponding to a transmission range of the FDD communication system. A TDD base station is provided within the TDD communication system for communicating with the mobile unit in time division duplex mode, and for generating a pilot search signal corresponding to a transmission range of the TDD communication system. A mobile unit transmits information via a current one of the FDD and TDD communication systems while moving towards the other, or targeted one of the FDD and TDD communication systems. The mobile unit includes a receiving unit to receive the first and second pilot search signals, and to initiate a handoff from the current communication system to the targeted communication system when the pilot search signal from the targeted communication system has reached a predetermined threshold level. The mobile unit further includes a dual-transceiver to synchronize concurrent communications between the mobile unit and the current and targeted communication systems, and to concurrently communicate the information with the current communication system and the targeted communication system upon synchronization. The dual-transceiver includes a burst mode capable of increasing a data rate between the mobile unit and the current and targeted communication systems during the handoff to concurrently support communication with both the current and targeted communications systems.
In accordance with another aspect of the invention, a method for maintaining a connection between a frequency division duplex (FDD) communication system and a time division duplex (TDD) communication system during handoff of a communication unit from one communication system to the other is provided. The method includes transmitting a first portion of a communication frame to one of the FDD and TDD communication systems, and transmitting a second portion of the communication frame to the other one of the FDD and TDD communication systems. At least one of the first and second portions of the communication frame is transmitted at an increased data rate. The communication occurs with both the FDD and TDD systems during the transmission of the communication frame, and the communication occurs in a time equivalent to the time required to transmit the communication frame to either of the FDD and TDD systems when no handoff is occurring.
In accordance with yet another embodiment of the invention, a method is provided for controlling information transmissions between a communications unit and both frequency division duplex (FDD) and time division duplex (TDD) communication systems. The communication unit is transmitting information via a first one of the FDD and TDD communication systems, and a cell identification signal is generated which corresponds to a transmission range of the other one of the FDD and TDD communication systems. Substantially simultaneous communication is initiated between the communications unit and both the FDD and TDD communications systems when the communications unit acknowledges the cell identification signal and is thereby in the coverage area of both the FDD and TDD communications systems. The substantially simultaneous communication between the communications unit and each of the FDD and TDD communications systems is maintained by simultaneously communicating at least a part of the information with each of the FDD and TDD communications systems.
The above summary of the present invention is not intended to describe each illustrated embodiment or implementation of the present invention. This is the purpose of the figures and the associated discussion which follows.